In present communication systems with multiple mobile users utilizing each base station, power control is needed in order to keep interference levels at minimum in the air interface while at the same time providing a required quality of service.
The so called Transmit Power Control (PC or TPC) functionality is crucial for interference limited cellular systems, such as WCDMA, GSM systems and 3G, where more high-powered mobiles interfere with weaker mobiles within one cell. If there were no mechanism for mobile stations to be power controlled to the same power level at a base station, a higher powered mobile station could easily overshout a weaker mobile. Power control adjusts the transmit power between a transmitter e.g. base station and a receiver e.g. mobile unit in order to maintain a certain quality while using no more transmit power than is absolutely necessary. This reduces the interference caused by the transmitter to other receivers. Also, the battery power for the mobiles is conserved.
In systems utilizing Code Division Multiple Access (CDMA), known power control functionalities that operate on the so called Signal to Interference Ratio (SIR) include Outer Loop Power Control (OLPC) and INner loop Power Control (INPC). OLPC is responsible for compensating for channel or link variations by adjusting the SIR target for the INPC based on the block error indicator (BEI) checked by a Cyclic Redundancy Check (CRC), which is sensitive to the accuracy of instantaneous quality measurements.
The INPC subsequently compares the estimated SIR at the receiver with the SIR target and adjusts the transmitted power accordingly. If the estimated SIR is higher than the SIR target a Transmit Power Control (TPC) command to lower the transmit power is signaled to the transmitter and vice versa if the estimated SIR exceeds the SIR target. However, for a coding block that experiences a multi-state channel, that the geometric average of SIR reaches the SIR target is equivalent to the decoding quality reaches the BLER target only when the neighboring Transmission Time Intervals (TTIs) have quite similar SIR statistic distributions. Therefore, known INPC is not optimal when neighboring TTIs have different SIR statistic distributions. This problem is more serious when channel various significantly during one.
In addition, in the cases of link-adaptation, the modulation and coding scheme (MCS) varies from one TTI to another. As a result, the SIR target based on the measurements of previously received will not always lead to the BLER target requirement for the current TTI. The same problem exists in AMR in a GSM system.
There are some known methods aiming at adjusting the INPC target based on a quicker quality measurement. In [1, 2, 3], two outer-loops have been used to keep the fast quality measurement reliable, with the slowest loop correct the quicker quality measurement based on a long-term quality measurement to keep the system stable. In [4] the attempts is made to shorten the FER measurement time and increase the measurement accuracy for the outer-loop power control by making use of the measurements of the other stronger channel or the other continuous transmitted channel, called associated channel.